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The Effects of 120KHz Ultrasonic Frequency on the Reliability of Fine Pitch Gold Wire Bonding

Keywords: wire bonding, high frequency, 120 khz

Gold wire bonding typically uses 60KHz ultrasonic frequency. Studies have been reported that increasing ultrasonic frequency from 60KHz to 120KHz can decrease bonding time, lower bonding temperature, and/or improve the bondability of Au metallized organic substrates. This paper presents a systematic study of the effects of 120KHz ultrasonic frequency on the reliability of fine pitch gold wire bonding. Two wire sizes, 1.0 mil and 0.7 mil in diameter, were used. The gold wires were bonded to metallized pads over organic substrates with five different metallization. The studies were carried out using a thermosonic ball bonder that is able to easily switch from ultrasonic frequency from 60KHz to 120 KHz by changing the ultrasonic transducer and the ultrasonic generator. Bonding parameters were optimized through design of experiment methodology for four different cases: 60KHz with 1.0 mil wire, 60KHz with 0.7 mil wire, 120KHz with 1.0 mil wire, and 120KHz with 0.7 mil wire. The integrity of wire bonds was evaluated by the wire pull and the ball bond shear tests. With the optimized bonding parameters, over 2,250 bonds were made for each frequency and wire size. The samples were then divided into three groups. The first group was subjected to temperature cycling from -55°C to +125°C with one hour per cycle for up to 1000 cycles. The second group was subject to thermal aging at 125°C for up to 1000 hours. The third group was subject to humidity at 85°C/85% RH for up to 1000 hours. The bond integrity was evaluated through the wire pull and the ball shear tests immediately after bonding, and after each 150, 300, 500, and 1000 hours time interval in the reliability tests. The pull and shear data are then analyzed to compare the wire bond performance between different ultrasonic frequencies.